Oxford
Feb. 3d 1852
Dear Sir
I have lately, from circumstances, had my attention recalled to the subject of your pamphlet <1> on Foucault’s experiment. I believe when I wrote <2> to you before on the subject I was inclined to differ from you as to the probable success of the experiment you propose. I was led to think that it would succeed from the analogy which subsisted between it and that described by M. Gargot[?]. I have however subsequently been convinced that those mathematicians are right who consider that there must have been some fallacy in that experiment – and in Consequence I now entirely agree with you that the experiment imagined of a bar freely suspended & balanced horizontally would not end in any apparent deviation due to the earth’s rotation.
This experiment seems to be the same in principle as that proposed by M. Baudrimont <3> (Comptes Rendus No 8. 1851) although this last is complicated by the influence of the torsion of the thread. It was objected to by some on that account – But there seems to me another reason applying to both. viz. that that [sic] the theory of Binet <4> & Foucault cannot be applicable to these cases inasmuch as that theory essentially involves the assumption that the body has an impressed motion given to it. it cannot therefore apply to a body at rest. If so I think your objection to the proposed experiment would not Constitute any ground for rejecting the theory of Foucault’s result.
Should you be giving any attention to the subject again I should feel obliged by hearing whether this view of the matter appears open to any objection & remain
Very sincerely yours
Baden Powell
Notes:
1. Remarks on M. Foucault’s Pendulum Experiment by H.F. Talbot, privately printed by Cox and Wyman, London, 1851. In March 1851 Jean Bernard Léon Foucault (1819–1868), French physicist, suspended a metal ball, weighing 28 kg, in a wire from the dome of the Panthéon in Paris. The ball was set in a pendulous motion, and over a span of hours it would exhibit a slow rotation in the direction of the pendulous motion, but what seemed to be the gradual rotation of the direction of the pendulous motion would actually be the rotation of the earth in space. In his paper WHFT suggested a different experiment, in which a horizontal bar balancing on a vertical bar would have to revolve within the span of 24 hours if Foucault’s reasoning was true. WHFT was convinced that this experiment would fail and thereby prove Foucault wrong, and the experiment would have failed, but perhaps not for the reasons WHFT thought. WHFT’s experiment would fail because he operated with an object, which was at rest with respect to the earth, whereas Foucault operated with an object, or rather a movement, which was at rest with respect to the frame defined by the stars. In the defence of WHFT only the fewest contemporary observers perceived this difference. [See Doc. No: 06438; see also Tobin, William, The Life and Science of Léon Foucault: the man who proved the earth rotates (Cambridge and New York: Cambridge University Press, 2003), pp. 133–172.].
2. See Doc. No: 06437.
3. Possibly E. A. Baudrimont (1806–1880), French scientist.
4. Jacques P.M. Binet (1786–1856), professor of mechanics at École Polytechnique and professor of astronomy at Collège de France. Binet was on the commission appointed by the Académie des Sciences to explain Foucault’s pendulum experiment.